Molecular Orbital Diagrams for TM Complexes with

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1 olcular Orbital Diagrams for T Complxs with p-intracting igands CO and othr iso-lctronic p-accptor s CO s ar vry usful for charactrising th amount of back donation in othr s o th vibrational IR spctrum of CO, and in particular th pak associatd with th C-O strtch is vry snsitiv to th amount of back donation o as othr s "tak" som of th finit amount of lctron dnsity, backdonation into CO dcrass and th CO vibrational pak movs to lowr wavnumbr this is a vry qualitativ xplanation, how can w quantify th amount of forward and back donation a contributs? o carry out a calculation and prform a population analysis! This tlls us how much of th lctron dnsity is on th mtal and how much is on th, and vn which orbitals it nds up in Tabl 1. o O + is on of th strongst p-accptors, but s-donation is poor o C - is a poor p-accptor howvr it is a strong s-donor o w can dduc that if th nrgy of th p* orbitals lin up wll with th T daos, this tnds to man that th s-orbitals hav a poor match, and vic vra o th carbonyl has an almost optimal orbital alignmnt and is a strong s-donor AD good p-accptor. w know that along with a small diffrnc in FO nrgis (D) a good ovrlap (S ij ) is rquird for a strong intraction o CO has a larg contribution on th O atom for th bonding p-fo and a vry small contribution on th O for th antibonding p*-fo o this has th ffct of substantially strngthning th dao-p*-accptor O ovrlap, Figur 2 o othr homonuclar diatomic s do not hav this advantag, for xampl 2 and O 2 hav qual orbital contributions, which rduc th dao-p*-accptor intracton OC CO CO CO =CO,C -,O + Figur 1 (CO) 4 complx complx s- donation p- accptanc F(CO) 4 C F(CO) 4 CO F(CO) 4 O Tabl 1 Data from th CDA analysis for quatorial C -, CO and O + s 1 vry good ovrlap mdium ovrlap π-accptor O π-accptor O CO O 2 π-donor O π-donor O Figur 2 orbital siz mattrs poor ovrlap mdium ovrlap 1 Y. Chn, Hartmann and G. Frnking, Z. Anorg. Allg. Chm., 2001, Vol 627, p985 Hunt / ctur 8 1

2 Othr p-intracting s X 2 s with multipl bonds can intract nd-on or sid-on, Figur 3, nd on is gnrally favourd othr s with multipl bonds can intract sid-on via thir p- FOs, including alkns and alkyns, Figur 3 it is important to distinguish btwn th FOs and th classification of th -mtal intraction, Figur 4 o σ π and δ rfr to rotation about th local bond o in th this is th E-E bond, in th complx this is th - bond how to formally dscrib th bonding (and hnc oxidation stat of th mtal) bcoms problmatic, Figur 5 o if thr is a larg amount of back-donation th - bond is vry strong and th intrnal bonding is rducd o should sid on coordinatd complxs b considrd as cyclic or as p-dnsity donating to a cntr? O thory provids part of a dscription of bonding, howvr, othr considrations must b takn into account, in particular s- rpulsion within p-bonding s can b larg and dstabilis coordination. In-Class Activity draw and annotat bonding and antibonding FOs for th p and p* orbitals of trans-( s ) 4 (CO) 2, th first on shown in Figur 6 H 2 C CH 2 CH CH Figur 3 othr typs of p-intraction with a T π -FO π - intraction π -FO δ - intraction Figur 4 distinguish clarly th intrnal vs th - intractions Figur 5 formal classification of th bonding bcoms difficult O C C O in- π-bonding - π-bonding in- π-bonding ovrall this FO is bonding Figur 6 trans-( s ) 4 (CO) 2 and on of th p-fo combinations Hunt / ctur 8 2

3 T-O Diagrams for two p-donor or two p-accptor s complxs can hav mor than on p-donor (( s ) 4 Cl 2 ) and mor than on p-accptor (( s ) 4 (CO) 2 ), thy can also hav on of ach typ (( s ) 4 (CO)Cl), or includ s that bind sid-on (( s ) 4 (HCCH) 2 ) T complxs can hav a vry larg rang of bonding typs and mods and undrstanding th complx bonding in ral molculs must b built up from an undrstanding of th simplr bonding intractions thnral procss is always th sam o to start with th undrlying octahdral drivd s-framwork o work out th rducd symmtry labls o add in th additional orbital intractions o ths diagrams ar complx, do a rough diagram first! for xampl th nrgy diagram for two p-accptor s can b built up o first th symmtry is dtrmind as D 4h, th axial systm dfind and th symmtry lmnts locatd and th orbital symmtris ar dtrmind for th s-framwork mtal and orbitals, Figur 7 z C 4 C 2 σ v C 2 y x σ d C 2 axis dfinition D 4h σ h point group and symmtry lmnts a 2u u O h D 4h D 4h symmtry mtal daos symmtry σ-fos Figur 7 complx and orbital symmtry o th nxt stp is to work on th additional componnts, th p p orbitals, which form a rang of FO combinations (s th arlir in-class problm!) o th symmtry of ths Os is asily gussd by analogy to axial or dao symmtry or altrnativly by knowing that thy must b dgnrat and xamining thir transformation undr invrsion. o bcaus ths sts of p p orbitals ar wll sparatd thr is almost no splitting and thy ar ssntially dgnrat. Hunt / ctur 8 3

4 I first drw in th black lins which mark th s-framwork, Figur 8 w can us our knowldg of th simpl CO FO nrgy positioning to locat th orbitals on th nrgy diagram, nxt I addd in th p and p* FOs o w hav twic th dgnrat sts bcaus thr ar two CO s o in CO th p-fos ar dpr in nrgy and rmain nar th s-fos (blu) o in CO th p*-fos li clos to and abov th mtal daos (pink) thn combin FOs of th sam symmtry, combining orbitals which ar clos in nrgy first. sktch out th Os around th d-manifold of orbitals (thos important for th complx bonding and ractivity) 4p u a 2u 4s u 3d π -basd Os "dz 2 " & "dx 2 -y 2 " π-basd Os σ-basd Os u a 2u u 3d u π -basd Os "dyz" & "dxz" Figur 8 doubl p-accptor nrgy lvl diagram and ky Os u π-basd Os Hunt / ctur 8 4

5 Summary Diagram gnratd aftr a studnt rqust in 2014! "in-": antibonding "btwn-": antibonding long rang "btwn-" intraction: small splitting w "in-" intraction not possibl for pi-donor rough positioning of T daos (this will vary slightly dpnding on th spcific mtal and s involvd) π FOs long rang "btwn-" intraction: small splitting π FOs "in-": antibonding "btwn-": bonding π FOs daos "in-" intraction lowrs nrgy FO π FOs "in-": bonding "btwn-": antibonding b 1 b 1 b 1 b 1 FOs du to th sigma part of th s "in-": bonding "btwn-": bonding "in-" intractions ar strongr than "btwn-" intractions -accptor -accptor two π-accptor -accptor singl π-accptor -donor singl π-donor -donor -donor two π-donor OTE: symmtry labls ar for C 4v and will chang as th symmtry of molcul changs du to th changing s Figur 9 D oct for singl and doubl p-accptor complxs Hunt / ctur 8 5

6 Octahdral Complxs with six p-donor or six p-accptor s complxs with six p-donor or six p-accptor s ar common, ths molculs rtain octahdral symmtry (O h ) complxs that form with p-donor s includ halid salts [X 6 ] n-, for xampl: [ncl 6 ] 3-, [CoCl 6 ] 3-, [RhCl 6 ] 3- [if 6 ] 4-, [CoF 6 ] 3- and [FF 6 ] 3-. Complxs that form with p-accptor s includ CO complxs (CO) 6 and C- complxs [(C) 6 ] n-, for xampl: [F(C) 6 ] 3-. th s-framwork rmains th sam for p-donor s, thr ar now 12 p p -AOs contributing to th FOs (a p x and p y orbital on ach of th six s). o th symmtry for ths can b complicatd to work out but rsolvs into a st of t 1g + + +t 2u FOs (on of ach t symmtry in O h ) o that is 4 groups of triply dgnrat orbitals (making 12 FOs in total) o th ky p-donor FOs and th rsulting bonding and antibonding Os ar shown in Figur 10 and on of ach st ar shown for th p- accptor, Figur 11 o I don t xpct you to b abl to rproduc ths complx Os!, but you should now thir symmtry labls for th O h nrgy diagram. FOs "dxz" "dyz" bonding Os dominatd "dxy" "dxz" "dyz" antibonding Os dao dominatd "dxy" Figur 10 Ky Os for 6 whr is p-donor FO bonding O dao dominatd antibonding O dominatd Figur 11 Ky Os for 6 whr is p-accptor Hunt / ctur 8 6

7 w know whr th p-donor FOs ar positiond, around th sam plac as for th singl or two p-donor FO, Figur 12a o th t 1g + + t 2u do not hav th corrct symmtry to intract with th mtal and rmain non-bonding, I hav rprsntd ths 9 FO as a solid block on th nrgy diagram (i a band of FOs!) o th FOs ar kpt sparat as ths do hav th corrct symmtry to intract with th mtal daos. 4p 4p 2 nd yar cours "T and Organomtallic Chmistry" 4s Δ oct mtal-basd Os 4s mtal-basd Os Δ oct t 1g t 2u 3d 3d mtal-basd Os t 1g t 2u t 1g t 2u Figur 12 nrgy diagram 6 whr (a) is p-donor and (b) is p-accptor an analogous procdur is followd for th p*-accptor FOs (Figur 12b), thr ar an additional 12 p*-fos contributing to th FOs. Th symmtry for ths also rsolvs into a st of t 1g + + +t 2u FOs w know whr th p-accptor FOs ar positiond, around th sam plac as for th singl p-accptor FO o again th t 1g + + t 2u do not hav th corrct symmtry to intract with th mtal and rmain non-bonding and I hav rprsntd ths 9 FO as a solid block on th nrgy diagram. o th FOs ar kpt sparat as ths do hav th corrct symmtry to intract with th mtal daos. Hunt / ctur 8 7

8 Th octahdral splitting paramtr for th six s-donors th daos ar non-bonding and th ar antibonding (with rspct to th s), this is th origin of th - splitting pattrn you hav bn told to us, now you know whr it coms from! with six p-donors th Os ar all antibonding, th nrgy gap btwn th and Os is rducd and hnc D oct is small rlativ to ( s ) 6 complx. Highly symmtry complxs with six p-donor s hav a small D oct with six p-accptors th Os ar all bonding and ar stabilisd incrasing th nrgy gap btwn th and Os, D oct is incrasd. Highly symmtry complxs with six p-accptor s hav a vry larg D oct notic that thr now also xists a manifold of Os within th rgion spannd by D oct (th non-bonding p* Os) ths ar almost always ignord in txt books! Othr typs of intraction to considr all 6 σ-donor s in an octahdral complx do not ncssarily nd to b th sam you should b abl to dscrib th O diagram for a systm in which th two axial σ-donor s ar of a diffrnt typ from th quatorial σ-donor s, Figur 13 ' ' Figur 13 forming an intrmdiat O diagram for th fragmnt orbitals p-donor and p-accptor s can add in mor than on coordination sit around th mtal, you should b abl to dscrib th O diagram for 2 axial p-accptor or p-donor s (i in a trans configuration) in th tutorial for ctur 7 you considrd a squar planar complx with 4 σ- bonding s, howvr ths could b 4 p-donor or p-accptor s. Exampls of squar planar complxs includ: PtBr 2 (PPh 3 ) 2, PtCl 2 (H 3 ) 2, [i(p 3 ) 4 ] 2+, [i(c) 4 ] 2- and [PdBr 4 ] 2-. of cours th all 6 p-donor (or p-accptor) s in an octahdral complx do not nd to b th sam, and thr can b a rduction in symmtry. For xampl: trans and cis [FCl 2 Br 4 ] 4-, [ROCl 5 ] - most complxs contain a mix of s-donor, p-donor and p-accptor s, in this cas th final O diagram can b xtrmly complx, and contains componnts from all th typs of diagram xamind so far. Exampls of mixd complxs includ FCl 2 (OH 2 ) 4, [FCl 2 n 2 ] + and Rh(CO)(H)(PPh3) 3 Th Ral Thing! Os from a ral calculation Hunt / ctur 8 8

9 Ky Points: b abl to discuss back-bonding and orintation (sid-on or nd-on) in rlation to th diffrnt orbital ovrlap and th nrgy match btwn σ- donor π-donor and π-accptor orbitals b abl to draw th nrgy lvl diagram for a T complx with sigmabonding s and on or two (trans) p-donor or p-accptor s. b abl to draw nrgy lvl diagrams for octahdral and squar planar transition mtal complxs with all π-donor and π-accptor s b abl to draw and dscrib th important Os for ths diagrams b abl to discuss ky faturs of ths diagrams, spcially faturs rlating to th charactr of th Os and D oct. Slf-Study Problms / Exam Prparation Us O diagrams and spcific xampls to xplain how a vry larg D oct could b obtaind Explain (mploying an nrgy diagram including ky Os) why H 3 gnrats a largr D oct than H 2 O in th following complxs; [Cr(H 3 ) 6 ] 2+ D oct 21,600 cm -1 and [Cr(OH 2 ) 6 ] 2+ D oct 17,400 cm -1 2 can intract ithr sid-on or nd-on, howvr nd on coordination is almost xclusivly found, rationalis why 2 prfrs to coordinat nd-on basd on th p*-fo intractions with a T Construct th O diagram for 2 10 whr =σ-donor, nsur you considr th formation of a quadrupl - bond Hunt / ctur 8 9

Molecular Orbitals in Inorganic Chemistry

Molecular Orbitals in Inorganic Chemistry Outlin olcular Orbitals in Inorganic Chmistry Dr. P. Hunt p.hunt@imprial.ac.uk Rm 167 (Chmistry) http://www.ch.ic.ac.uk/hunt/ octahdral complxs forming th O diagram for Oh colour, slction ruls Δoct, spctrochmical